1. Field of the Invention
The present invention relates to technology that removes foreign substances that adhere to an optical member provided at the front of an image sensor and the periphery thereof in an image capturing apparatus such as a digital camera.
2. Description of the Related Art
An image capturing apparatus such as a digital camera that captures an image by converting an image signal into an electrical signal receives a photographing light beam with an image sensor, converts a photo-electric conversion signal that is output from the image sensor into image data, and records the image data on a recording medium such as a memory card. A CCD (Charge Coupled Device) or a C-MOS (Complementary Metal Oxide Semiconductor) or the like is typically used as an image sensor.
In this kind of image capturing apparatus, an optical element such as an optical low pass filter (hereunder, referred to as “LPF”) or an infrared cut-off filter is often disposed between a photographing lens and an image sensor. If a foreign substance such as dust adheres to the cover glass of the image sensor or to the surface of these optical elements, the shadow of that foreign substance will be transferred onto the solid-state image sensor.
These kinds of foreign substances are in fact very minute particles measuring less that several tens of μm. It is considered that the foreign substances are caused by dust or the like entering the camera from outside when changing the lens, or by the generation by abrasion (accompanying operation of the shutter or mirror inside the camera) of powder from resin or the like that is a structural member of the camera interior.
There are cases in which, in particular, dust generated by these kinds of causes penetrates into the area between a protective cover glass of the solid-state image sensor and an infrared cut-off filter or LPF or the like disposed in front of the cover glass. In such cases, the camera must be disassembled in order to remove the dust. It is therefore extremely desirable to adopt a sealed structure in order to prevent dust penetrating between the cover glass of the solid-state image sensor and an optical filter.
However, when dust adheres to the surface of a side facing a solid-state image sensor of an optical filter as well as an opposite side, it is difficult to remove the dust since the dust is very small. Further, when the position at which the dust adheres is in the vicinity of the focal plane, the problem remains that the dust forms a shadow that is clearly reflected on the solid-state image sensor.
In particular, in recent years the trend of capturing images that have a large number of pixels is also proceeding in the field of lens-interchangeable single-lens reflex digital cameras, and 135 format cameras of this type are commercially available in the 10 million pixel class. As high definition images are obtainable accompanying this trend towards capturing images that have a large number of pixels, demand is increasing from users who wish to enlarge the display of captured high resolution images to check the images. Since the shadows of foreign substances are all the more conspicuous when a user performs this kind of enlarged display, foreign substances that adhere to the vicinity of the focal plane of the photographing lens are being seen as more of a problem than in the past.
In particular, in single-lens reflex digital cameras in which it is possible to exchange lenses, mechanical operating units such as shutters and quick return mirrors are disposed in the vicinity of the image sensor. Foreign substances such as dust generated by these operating units sometimes adheres to the image sensor or to a low pass filter. Dust or the like can also enter inside the camera main unit through the lens mount opening when exchanging lenses.
In order to solve this problem, Japanese Patent Laid-Open No. 2002-204379 discloses a single-lens reflex digital camera having a photographing optical system that forms an optical image of an object, a photo-electric conversion element that converts the optical image into an electrical signal, an optical element that is disposed between the photographing optical system and the photo-electric conversion element, and a vibration means comprising a piezo-electric element that causes the optical element to vibrate. By causing the optical element to vibrate using the piezo-electric element, a foreign substance such as dust particles that adheres to the surface of the optical element can be removed.
The single-lens reflex digital camera disclosed in Japanese Patent Laid-Open No. 2002-204379 is configured to remove dust or the like that adheres to the surface on the photographing lens side of the optical element that is disposed at the front surface of the photo-electric conversion element by causing it to fly off the optical element surface by vibrating the optical element using the vibration means. However, since a shutter is normally provided close to the front surface of the optical element in a single-lens reflex digital camera, some dust or the like that is knocked off the optical element reattaches to the surface of a group of shutter blades of the shutter that cover the optical element front surface.
When an exposure operation starts in this state, there is a possibility that the dust or the like that is adhered to the surface of the group of shutter blades will be knocked off from that surface when the shutter blades are driven, and will again adhere to the optical element surface. There is thus the problem that in some cases removal of dust from the optical element surface is incomplete.